@unpublished{FedosovSchulzeTarkhanov2008,
  author  = {Boris Fedosov and Bert-Wolfgang Schulze and Nikolai Nikolaevich Tarkhanov},
  title   = {On the index formula for singular surfaces},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(1997) 31},
  year    = {2008},
}